<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style>
			.main{
				background:red;
				display: flex;
				justify-content: center;
			}
			.a{
				width:200px;
				height:200px;
				background:blue;
			}
		</style>
	</head>
	<body>
		<div class="main">
			<div class="a"></div>
		</div>
	</body>
</html>